1. Field of the Invention
The present invention relates to a mechanism for moving lenses in a zoom lens assembly, and more particularly to a mechanism for moving a part of the zooming lens groups in response to a focusing operation, keeping the remaining part of the zooming lens groups at rest.
2. Description of the Prior Art
Prior zoom lenses are generally constructed such that the foremost lens group is moved along the optical axis for focusing. Where such a lens construction is incorporated in zoom lenses with high zoom ratio ranging to wide-angle, however, the brightness at the edge of the image field is reduced to a large extent and for this reason the lens diameter should be increased. Another problem is that aberrations differ widely dependent on the distance to the object. To solve the above problems, there has been proposed, for example by Japanese patent laid-open publication No. Sho. 57-37308 published on Mar. 1, 1982, a zoom lens having lens groups movable for zooming with part of the lens groups being also movable for focusing. With the proposed zoom lens, the lens groups are drivable by cam grooves provided in a member movable upon zooming, and some of the lens groups are adjustable by cam grooves provided in another member movable upon focusing. However, the proposed lens construction is complex since the separate members are necessary which are movable upon zooming and focusing, respectively, and a complicated holder mechanism is required for allowing those lens groups which are movable in both zooming and focusing modes to operate separately in those modes. In the zoom lenses with high zoom ratio ranging to wide-angle, the lens elements should be installed with high accuracy. With the complex prior art arrangement, however, no desired high accuracy can be achieved as the lens groups are increasingly inclined and off center.
In case the lens groups that are movable in the focusing mode are located close to the front end of the lens assembly, the members used for focusing the lens groups are also disposed close to the front end of the lens assembly. Where such lens is adapted to effect focusing by an automatic focusing mechanism positioned in a camera body as proposed in recent years, certain shortcomings arise. Specifically, the focusing members located close to the front end of the lens assembly are remote from the camera body, and a connector by which the focusing members and the automatic focusing mechanism in the camera body are operatively interconnected takes up a large space. In addition, the accuracy with which driving movement is transmitted from the automatic focusing mechanism to the focusing members is low. It is therefore preferable that the focusing members be located close to the rear end of the lens assembly. Nevertheless, no accurate and sufficient driving force can be transmitted from the rearward located focusing members to the focusing ens groups if the focusing lens groups remain positioned close to the front end of the lens assembly.